Monday, 14 September 2015

Stable Earth?

It is due to the remarkable progress in climate science that we are now able to have this knowledge.  It is no coincidence that this knowledge comes at the same time as many other advances in science as they are ultimately connected through the same laws of physics and chemistry.

When we look back through eons of paleo-climate there are two opposing questions that come to mind.

 How can the Earth have remained stable enough to maintain life? This is in spite of solar activity increasing by nearly 5% over the last five or 6 hundred million years over a period of time known as the phanerozoic when life evolved large enough to view the fossilized remains without the aid of microscopes. (This 5% is about 50 times greater than the plus or minus swings that we associate with a typical 11year solar cycle.)
How can Earth have gone through large swings in temperature over shorter time scales? These swings would make life for human civilization as we know it impossible.


The observations that the Earth’s temperatures have remained within limits leads us to the first question above. This further correctly indicates that there must be some stabilizing effects, or negative feedbacks, but incorrectly lead some to believe that these stabilizing effects imply there are no concerns and therefore present day greenhouse gases can have no effect.  These limits are not constrained enough for human civilization.

The observations that the Earth has gone through massive swings correctly lead us to see that there are dangerous positive or amplifying feedbacks due to some factors, or forcings, that have destabilized the climate on many occasions. Without an amplifying feedback the magnitudes in the swings can’t be numerically explained by the forcings alone. (On much shorter time scales, that are only really resolvable in more recent history, we also see swings in the climate over periods of about hundred thousand years, although not as extreme these also cannot be explained without some positive feedbacks at play. These swings are associated with the Milankovitch cycles due to the changes in the Earth’s tilt and orbit around the sun, but are not the swings that are discussed in here.)

Triggers and feedbacks can account for these changes.
If some large enough trigger (or forcing) for change occurs then positive feedbacks step in amplifying that change in the direction of the initial trigger, then on longer time scales negative feedbacks step in ultimately bringing the swing to an end.  This whole process may have been in the order of tens of millions of years. Eventually we get a trigger of the opposite sign; one that has almost been inevitable.
These triggers occur on a relatively short time scale at the onset of a new change in direction and it is unlikely to be a change of solar activity as the solar activity is remarkably constant over these short time scales. (We must take account of the differences of solar activity however when comparing different eras of climate.)

Positive forcing  >>>>>>> Positive feedbacks>>>>>>>>>>.negative feedback...........
Negative forcing  >>>>>>> Positive feedbacks>>>>>>>>>>.negative feedback

So a positive forcing would increase global temperatures, positive feedbacks would amplify this increase and eventually some negative feedback would ultimately limit this increase. Later a negative forcing would reduce temperatures, some positive feedback would amplify this temperature reduction and eventually some negative feedback would limit this reduction. 

Examples of forcings.
A positive forcing causing increased temperature could be a massive volcanic output of CO2.
A negative forcing could be..... reduced volcanic activity, increased albedo due to continents moving over the poles, increased weathering of rocks removing carbon dioxide or a combination of these with contributing changes in the Earth’s orbit and tilt.

Positive feedbacks:-
These would inevitably be at least..... oceans changing amount of CO2 released or absorbed, water vapour feedback and albedo changes.

What could the negative feedbacks be over these extremely long time scales?

When applying the laws of physics there is an automatic feedback effect included whereby the warmer an object is the more it releases heat limiting the final temperature reached. This is the same physics applied if for example one continuously adds heat to say an iron bar in a flame. It will ultimately reach a temperature whereby it is absorbing and emitting energy at the same rate. (This however works on all time scales and it is automatically included when one applies the Stefan- Boltzmann equation.)

Also greenhouse gases have been the control for the Earth’s temperature. On shorter time scales increased CO2 increases the temperature and this in turn increases the water vapour but this is a positive feedback....on longer time scales with increasing CO2 the resulting melting ice reduces the Earth’s albedo and further CO2 is released from the oceans. These are also positive feedbacks. On even longer time scales CO2 acts as a negative feedback.

So it is the greenhouse effect that could help us explain not only the extreme swings in temperature but also why the Earth has remained within limits (in spite of solar activity that gradually increases over eras of time.)

CO2 as a negative feedback over tens of millions of years.
The weathering of rocks removes CO2 from the air. This weathering increases when land is exposed or temperatures increases or when there is more rain.
When temperatures increase more ice melts, more rain falls and more weathering of rocks occur. Gradually over millions of years the atmospheric CO2 reduces as it falls behind CO2 emission from volcanoes.
When temperatures decrease more ice is formed and with lower temperatures, less rain and less exposed land all cause weathering to decrease. This allows CO2 to build up at a faster rate than the CO2 from volcanic activity.

Looking to the future.

Everything is NOT merely cyclic. There has never been this much CO2 in our atmosphere since humans have been on this planet and if we look further back hundreds of millions of years ago the solar activity was weaker. It will be many tens of millions of years into the future before an increasing sun will warm our planet noticeably but within just several decades we risk much more warming from rising CO2 with all the impacts warned by climatologists. How much more warming will mainly depend on how we react to the knowledge we presently have.

See also:-